Procedure for manufacturing fire-proof crucibles for steel-processing ladles

ABSTRACT

The procedure for manufacturing fireproof crucibles for steel-processing ladles, in which aluminum deoxidation is carried out for the total quenching of liquid steel, is such that a monolithically hardening fireproof mass with a proportion of silicon dioxide is placed between the inside wall of the ladle and a reusable plug that is installed therein. After the plug has been removed, the ladle is put into use again, at which time a layer that contains alumina builds up on the hot side of the fireproof crucible and a highly sintered layer of greater hardness forms behind this. It is intended that a procedure such as this should simplify the reestablishment of the fireproof lining when the limits of use are reached, in view of the use of a layer that contains alumina and the highly sintered layer located behind this on the inside wall of the crucible. To this end, after a number of charges have been processed, a second crucible is installed as an expendable crucible within the first crucible, which now forms a safety crucible, this being done with the help of an appropriately smaller plug; when this reaches its limits of use it is broken out and the layer is reinstalled.

The present invention relates to a process for manufacturing fireproofcrucibles for steel-processing ladles, in which aluminum deoxidation iscarried out in order to quench liquid steel.

BACKGROUND OF THE INVENTION

Crucibles that are of fire-proof bauxite and which are producedaccording to such known procedures form a highly sintered surface layeron their hot side during operation; this surface layer is based on themetallurgical process of aluminum deoxidation that is used to producecompletely killed steels, of the kind that are required, in particular,for continuous slab casting. The underlying chemical reaction is:

    4Al.sub.met +3 SiO.sub.2 solid =2Al.sub.2 O.sub.3 solid +3Si.sub.met

The metallic aluminum is present in the liquid steel, whereas thesilicon dioxide originates from the fire-proof bauxite mass. The layerthat contains the high alumina (Al₂ O₃), which is referred to as"furring" in professional circles, and an extremely hard sintered layerthat is located behind it, are formed on the hot side of the bauxitelining.

A disadvantage connected with crucibles that are manufactured andstressed in this way is the furring, Which increases with every chargeand which leads to constantly diminishing capacity of thesteel-processing ladle. If the lower limiting volume of the steel ladleis reached, all of the fire-proof crucible must be broken out, a taskthat is rendered extremely difficult because of the hardness of thesinter. For a considerable time, unsuccessful attempts have been made toremove the furring on the inner side of the crucibles by using pick-typetools; such attempts have been unsuccessful because when the sinter isbroken off, the crucible becomes brittle, at least, and as a rule itdisintegrates completely.

SUMMARY OF THE INVENTION

It is the task of the present invention to create a procedure of thetype discussed above, that uses the formation of the high-alumina layerand the highly sintered layer that is located behind it on the inside ofthe crucibles to simplify the reestablishment of the fire-proof liningwhen the limit of use is reached.

This problem has been solved by a procedure of the type described, usingthe distinguishing features.

It is essential for the present invention that the furring on the outersafety crucible, with the highly sintered layer, makes it possible tobreak out the expendable crucible without becoming damaged itself. Thereis no sintering of the fire-proof material of the expendable cruciblewith that of the safety crucible, so that the sintered layer of thesafety crucibles forms a boundary layer, and it is possible to work asfar as this when breaking out the expendable crucible.

It is not necessary to determine the precise number of charges that haveto be processed in order to form the safety crucible with sufficientfurring. In some instances, it is necessary to keep track of theoccurrence of the furring, in which case one can assume that 10 to 50charges should be completed with the first crucible prior to theintroduction of the expendable crucible.

It is a particular advantage if the ceramic of the expendable crucibleand, accordingly, the fire-proof substance that is used for this, issuch that, during operation, there is no complete sintering through thethickness of the wall of the expendable crucible. The expendablecrucible adjacent to the furring of the outer safety crucible has aboundary layer that is of significantly less strength than the adjacent,sintered boundary layer of the safety crucible, which means that asubsequent scaling or breaking off of the inner expendable crucible ismade easier and that damage to the safety crucible is all but precluded.Thus, the the reestablishment of the inner layer that forms theexpendable crucible can be carried out several times until, for safetyreasons, the first lining, which is to say the safety crucible, has tobe broken out and replaced by a new one.

A fireproof substance that has an identical or similar raw materialsbase and an identical or similar binding system as that used for thesafety crucible can be used for the expendable crucible so that aluminumdeoxidation for the complete quenching of the liquid steel can beeffected in the appropriate steel ladle. The useful limit is reachedwhen the effective volume of the ladle falls below the prescribedminimum as a consequence of furring. However, it is also possible to usea basic fireproof substance that contains little or no silicon oxide forthe expendable crucible. Such a basic expendable crucible of magnesiumoxide or of a mixture of alumina and magnesium oxide or dolomite andmagnesium oxide wears gradually. No furring forms on it because of itschemical composition, which contains very little silicon oxide. It ispossible to wear out the basic expendable crucible either completely orelse scale it or break it out after several charges, depending onoperating conditions or according to the wear pattern, after which onecan install a neutral expendable crucible in the safety crucible, asdesired. It is advantageous that one adjusts the ceramic of the basicfireproof substance for the basic expendable crucible such that in thesintered state it is softer than the safety crucible, so as to ensurethat the expendable crucible breaks out without any damage being done tothe safety crucible.

BRIEF DESCRIPTION OF THE DRAWING

The present invention will be described below on the basis of thedrawing appended hereto. The drawing is a diagramatic cross sectionthrough the wall of a steel-procedureing ladle with a fireproof lining.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The steel-processing ladle has a steel outer casing 1, on the inside ofwhich a permanent, insulating lining 2 is installed, optionally with aninsulating mat interposed between them. Taken as a whole, this resultsin a highly effective insulating, permanent lining on the inside of thesteel casing 1 of the ladle, and this serves to provide thermalinstallation.

First, a reusable steel plug (not shown in the drawing) is installed inthe ladle in such a way as to leave a gap of approximately 8-12 cmbetween the plug and the permanent lining. The steel plug is the firststeel plug that is needed, and it is of a greater diameter than thesecond steel plug that is used; this will be described in greater detailbelow.

A bauxite mass with a low cement content and an additional chemicalbinder with a conventional or exothermic binding system is inserted intothe gap between the steel plug and the insulating and permanent lining2. The following guide analysis applies to this, details being shown inpercentages by weight:

Al₂ O₃ approx. 78-88%

SiO₂ approx. 4-88%

TiO₂ approx. 1-3%

Fe₂ O₃ approx. 1-1.5%

CaO approx. 0.5-1%

Remainder: Alkalis and other oxides

After this fireproof lining has bonded, it results in the outerfireproof crucible of the steel ladle that subsequently forms the safetycrucible.

As soon as the first fireproof lining has bonded sufficiently, thereusable first steel plug is withdrawn from the ladle. The firstcrucible is then dried and heated; to this end, the steel ladle isplaced in the steel plant and 10 to 50 charges of alumina deoxydizedsteel are processed in it.

The first crucible is now sintered, and has very thin furring thatadheres firmly to the inner surface of the crucible. Immediately beneaththis there is a layer of mass that is approximately 0.1 to 5 cm thickand this is more strongly sintered and is harder. Deeper within thefireproof material, closer to the steel casing 1, there is almostcompletely unmodified bauxite lining 3 without any infiltration, and ahighly sintered layer 4 that is of greater hardness is located on thisand this layer 4 is covered to the outside by an alumina furring 5 thatconsists of the products of the aluminum deoxidation.

After the ladle has cooled down, a second reusable steel plug that is ofappropriately smaller diameter compared to the previously used plug isso inserted into the safety crucible 3-5 that has been formed by thefirst crucible that a gap of suitable size is left between the surfaceof the plug and the alumina furring 5. In practice, a gap of 5-6 cm ormore is left; the gap should be no smaller than 4 cm.

Once again, a fireproof mass is inserted into this gap in order to forma so-called working lining, which can also be referred to as a workingcrucible or expendable crucible. After this working lining-has bonded,the second reusable steel plug is withdrawn from the ladle and theworking lining is dried and then heated. Subsequently, the ladle is onceagain returned to the steel works where, after sintering, one obtains anexpendable crucible that again has a layer 6 that is of an almostoriginal largely unsintered fireproof material, a highly sintered layer7 that is of greater hardness, and aluminum furring 8 that is formedfrom the products of the aluminum deoxidation. Up to one hundred chargesor more can be processed with this expendable crucible 6-8, and when theuseful limit is reached, the expendable crucible is broken out. Whenthis is done, it is easy to orientate oneself with the hard sinteredlayer 4 of the safety crucible that is covered with the alumina furring5, in front of which there is a relatively soft layer 6 that is thuseasy to break out and which is of almost original fire proof material.Thus, the relatively soft layer 6 forms a nominal break point whenseparating out the worn-out expendable crucible.

The expendable crucible 6-8 that is shown in the drawing consists of afireproof mass with an identical or similar raw materials base as wellan identical or similar binding system, as in the safety crucible 3-5.In principle, in place of an expendable crucible of this kind, it isalso possible to use a basic expendable crucible that does not formfurring because of its chemical composition that contains little siliconoxide. Such an expendable crucible wears gradually, in which connectionhere, once again, the hard sintered layers 4 and 5 of the safetycrucible simplify removal of the rest of the expendable crucible. Forthe remainder, it is also possible to break out the basic expendablecrucible even before it reaches its wear limit, if the steel ladle is tobe provided with a new fireproof lining to form another, e.g., neutral,expendable crucible for other purposes.

I claim:
 1. A process for manufacturing fireproof crucibles forsteel-processing ladles, in which aluminum deoxidation is carried outfor total quenching of liquid steel, in which a monolithically hardenedfireproof mass that contains alumina with a proportion of silicondioxide is inserted between an inside wall of the ladle and a reusablesteel plug is installed therein forming a fireproof first crucible, theladle being used after removal of the plug, at which time a layer thatcontains alumina forms on a hot inner side of the fireproof firstcrucible, a highly sintered surface layer of greater hardness formingthereon, wherein after a number of charges, using an appropriatelysmaller plug, a second expendable crucible is formed inward of thefireproof first crucible whereby the second expendable crucible can bebroken out on reaching its limit of use, and a replacement secondexpendable crucible can then be reinstalled.
 2. The process as claimedin claim 1, wherein from one to fifty charges are processed with thefireproof first crucible, before the installation of the secondexpendable crucible.
 3. The process as claimed in claim 2, whereinceramic of the second expendable crucible is such that no completesintering takes place through the thickness of its walls duringoperation.
 4. The process as claimed in any one of the claims 1 to 3,wherein a fireproof mass with substantially same raw materials base aswell as substantially same manufacturing steps as those of the firstreusable crucible is used for the second expendable crucible.
 5. Theprocess as claimed in any one of the claims 1-3, wherein a basicfireproof mass that contains little or no silicon dioxide is used forthe second expendable crucible.
 6. The process as claimed in claim 5,wherein ceramic of the basic fireproof mass is such that when it is inthe sintered state it is softer than the first reusable crucible.